Rewrote bcfg2-crypt.

This adds several features and generally makes it work a lot more
smoothly by not trying to overoptimize and share too much code.

You can now brute-force decrypt a Cfg file (i.e., without specifying a
passphrase, it will try all of them).

You can also now write a Properties file with
`encrypted="passphrase_name"` attributes in place, and then encrypt
(and decrypt) it in one fell swoop.

Various other minor improvements.